The present invention relates to thermal barriers. More particularly, the present invention relates to thermal barrier extrusions utilized in providing insulated frames for doors and windows.
At the present time, almost half of the aluminum windows and doors sold in the United States utilize thermal barrier extrusions. Thermal barrier extrusions are typically aluminum extrusions used to make the various parts of windows and doors such as the sash, sill, threshold, and jamb. In addition they are used to make the frames for mounting window, spandrel, and door units. The thermal barrier extrusion generally provides a longitudinal channel body which defines a longitudinal U-shaped cavity into which is placed an insulating material, such as polyurethane or epoxy resin, to provide a thermal barrier for thermal isolation of the mounting.
The most common method for preparing thermal barriers involves pouring a fast setting polyurethane liquid casting compound into the U-shaped cavity of the extrusion. When the compound has hardened, the bottom of the thermal barrier extrusion is cut away or otherwise removed. Removal of the bottom of the thermal barrier extrusion results in a thermal barrier having two aluminum halves joined together by the polyurethane or other thermally insulating material. No longer is there a metallic, heat conducting bridge between the two halves. Rather, the two halves are separated by the polyurethane or other thermally insulating material. The thermally insulating material serves as both a thermal insulator and additionally as a connective element.
In spite of their popularity and obvious energysaving characteristics thermal barriers suffer from two major drawbacks or deficiencies. First, repeated temperature cycling of thermal barriers leads to longitudinal, end-to-end shrinkage of the thermally insulating material within the U-shaped cavity of the aluminum thermal barrier extrusion. The shrinkage results in the formation of gaps at the ends of the window or door frame extrusion. These gaps can cause water or air leakage into the building with subsequent damage or energy loss. Shrinkage of the thermally insulating material occurs due to a complicated process which is not completely understood. However, there is reason to believe that the problem is caused mainly by the large difference in coefficients of thermal expansion between aluminum and the thermally insulating material, such as polyurethane. For example, aluminum has a coefficient of thermal expansion of 1.3.times.10.sup.-5 per degree Fahrenheit while that of polyurethane ranges from 5 to 7.times.10.sup.-5 per degree Fahrenheit, depending on composition. With changes in temperature the polyurethane or other thermally insulating material typically expands or contracts more than the surrounding aluminum. This differential expansion and contraction of the thermally insulating material ultimately leads to shrinkage of the thermally insulating material relative to the extrusion. Since all thermal barriers are composite structures, they are inherently susceptible to this shrinkage problem. An effective solution to the problem is highly desirable.
A second problem experienced with some thermal barriers is insufficient impact strength. Aluminum and other metallic extrusions are relatively ductile materials which are relatively resistant to impact. Polyurethane resin and other typical thermally insulating materials are relatively brittle. As a result, the thermal insulation portion of the thermal barrier often splits and cracks when the thermal barrier is dropped, cut, sawed or run through a punch press. In addition, finished window and door assemblies may break during installation if not handled carefully. The splitting or cracking of the thermally insulating material is extremely undesirable since the material is not only functioning as insulation, but is additionally a connective element which holds the two aluminum halves of the extrusion together.
It is presently desirable to provide thermal barrier extrusions in which the thermally insulating material is mechanically bound or otherwise secured within the extrusion cavity to prevent or reduce longitudinal shrinkage of the thermally insulating material. Further, it would be desirable to provide thermal barrier extrusions in which the tendency of the thermally insulating material to split, crack or otherwise fracture is reduced.